1. Field Of The Invention
The invention relates to improved non-consumable electrodes, particularly for use in the production of aluminum in Hall-Heroult cells, and to a method for achieving a uniform current density on the electrode working surface.
2. Description Of The Prior Art
Aluminum is conventionally produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes. During the reaction, the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##STR1##
The problems caused by the use of carbon anodes are related to the cost of the anode consumed in the above reaction and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the fabrication of the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing troublesome workplace and environmental pollution problems. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon were consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered. More importantly, with no carbon consumed at the anode there would be no contamination of the atmosphere or the product from the impurities present in the coke.
Attempts have been made in the past to use non-consumable electrodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen and/or the cryolite bath. Ceramic compounds, such as oxides with perovskite and spinel crystal structures, usually have too high electrical resistance or are attacked by the cryolite bath.
Previous efforts in the field are disclosed in U.S. Pat. No. 3,718,550--Klein, Feb. 27, 1973, Cl. 204/67; U.S. Pat. No. 4,039,401--Yamada et al., Aug. 2, 1977, Cl. 204/67; U.S. Pat. No. 2,467,144--Mochel, Apr. 12, 1949, Cl. 106/55; U.S. Pat. No. 2,490,825--Mochel, Feb. 1, 1946, Cl. 106/55; U.S. Pat. No. 4,098,669--de Nora et al., July 4, 1978, Cl. 204/252; Belyaev+Studentsov, Legkie Metal 6, No. 3, 17-24 (1937), (C.A. 31 [1937], 8384) and Belyaev, Legkie Metal 7, No. 1, 7-20 (1938) (C.A. 32 [1938], 6553).
Of the above references, Klein discloses an anode of at least 80% SnO.sub.2, with additions of Fe.sub.2 O.sub.3, ZnO, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, V.sub.2 O.sub.5, Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or WO.sub.3. Yamada discloses spinel structure oxides of the general formula XYY'O.sub.4 and perovskite structure oxides of the general formula RMO.sub.3, including the compounds CoCr.sub.2 O.sub.4, TiFe.sub.2 O.sub.4, NiCr.sub.2 O.sub.4, NiCo.sub.2 O.sub.4, LaCrO.sub.3, and LaNiO.sub.3. Mochel discloses SnO.sub.2 plus oxides of Ni, Co, Fe, Mn, Cu, Ag, Au, Zn, As, Sb, Ta, Bi and U. Belyaev discloses anodes of Fe.sub.2 O.sub.3, SnO.sub.2, Co.sub.3 O.sub.4, NiO, ZnO, CuO, Cr.sub.2 O.sub.3 and mixtures thereof as ferrites. De Nora discloses Y.sub.2 O.sub.3 with Y, Zr, Sn, Cr, Mo, Ta, W, Co, Ni, Pd, Ag, and oxides of Mn, Rh, Ir, and Ru.
The Mochel patents relate to electrodes for melting glass, while the remainder are intended for high temperature electrolysis, such as Hall-Heroult aluminum reduction. Problems with the materials above are related to the cost of the raw materials, the fragility of the electrodes, the difficulty of making a sufficiently large electrode for commercial usage, and the low electrical conductivity of many of the materials above when compared to carbon anodes.
U.S. Pat. No. 4,146,438, Mar. 27, 1979, de Nora et al., Cl. 204/1.5, discloses electrodes comprising a self-sustaining body or matrix of sintered powders of an oxycompound of at least one metal selected from the group consisting of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminum, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and at least one electroconductive agent, the electrodes being provided over at least a portion of their surface with at least one electrocatalyst.
U.S. Pat. No. 3,930,967--Alder, Jan. 6, 1976, Cl. 204/67, discloses bi-polar electrodes made by sintering formed mixtures of SnO.sub.2, as a principal component, with small percentages of Sb.sub.2 O.sub.3, Fe.sub.2 O.sub.3 and CuO.
U.S. Pat. No. 3,960,678--Alder, June 1, 1976, Cl. 204/67, discloses a Hall-Heroult process using an anode having a working surface of ceramic oxide, wherein a current density above a minimum value is maintained over the whole anode surface to prevent corrosion. The anode is principally SnO.sub.2, preferably 80.0 to 99.7 wt. %. Additive oxides of Fe, Cu, Sb and other metals are disclosed.
U.S. Pat. No. 4,057,480--Alder, Nov. 8, 1977, Cl. 204/290 R, a divisional application from U.S. Pat. No. 3,960,678, relates to a ceramic oxide anode for a Hall-Heroult cell using a current density maintained above a minimum value over the contact surface of the anode. A protective ring is fitted over the three phase zone at the air-electrolyte-anode junction. Anode base material of SnO.sub.2, 80.0-99.7 wt. % is shown with additions of 0.05-2.0 wt. % of oxides of Fe, Cu, Sb and other metals as dopants.
U.S. Pat. No. 4,233,148--Ramsey et at., Nov. 11, 1980, Cl. 204/291, discloses electrodes suitable for use in Hall-Heroult cells composed of SnO.sub.2 with various amounts of conductive agents and sintering promoters, principally GeO.sub.2, Co.sub.3 O.sub.4, Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, MnO.sub.2, CuO, Pr.sub.2 O.sub.3, In.sub.2 O.sub.3 and MoO.sub.3.
Despite the efforts described above, preparation of usable electrodes for Hall-Heroult cells still has not been fully realized and no instance is known of any plant scale commercial usage. The spinel and perovskite crystal structures have in general displayed poor resistance to the molten cryolite bath, disintegrating in a relatively short time. Electrodes consisting of metals coated with ceramics using conventional methods have also shown poor performance, in that almost inevitably, even the smallest crack leads to attack on the metal substrate by the cryolite, resulting in spalling of the coating, and consequent destruction of the anode.
The most promising developments to date appear to be those using stannic oxide, which has a rutile crystal structure, as the basic matrix. Various conductive and catalytic compounds are added to raise the level of electrical conductivity and to promote the desired reactions at the working surface of the anode.
A major cause of the difficulties experienced with the use of conductive anodes having flat working surfaces in Hall-Heroult cells is the high current densities that exist at the edges and corners of the anodes. As a result, the operating life of these anodes is shortened by selective attack of these regions by the molten electrolyte bath. Regarding anodes having a protective surface covering, it has been accepted and common practice to utilize a material of very high electrical resistivity for the covering, compared to the resistivity of the protected material.